drm/amd/display: Fixed kernel test robot warning

Kernel test robot throws below warning ->

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:5349:5:
warning: no previous prototype for 'amdgpu_dm_crtc_atomic_set_property'
[-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:5349:5:
warning: no previous prototype for function
'amdgpu_dm_crtc_atomic_set_property' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:5373:5:
warning: no previous prototype for 'amdgpu_dm_crtc_atomic_get_property'
[-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:5373:5:
warning: no previous prototype for function
'amdgpu_dm_crtc_atomic_get_property' [-Wmissing-prototypes]

As these functions are only used inside amdgpu_dm.c, these can be
made static.
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -5342,7 +5342,7 @@ dm_crtc_duplicate_state(struct drm_crtc *crtc)
 }
 
 #ifdef CONFIG_DEBUG_FS
-int amdgpu_dm_crtc_atomic_set_property(struct drm_crtc *crtc,
+static int amdgpu_dm_crtc_atomic_set_property(struct drm_crtc *crtc,
 					    struct drm_crtc_state *crtc_state,
 					    struct drm_property *property,
 					    uint64_t val)
@@ -5366,7 +5366,7 @@ int amdgpu_dm_crtc_atomic_set_property(struct drm_crtc *crtc,
 	return 0;
 }
 
-int amdgpu_dm_crtc_atomic_get_property(struct drm_crtc *crtc,
+static int amdgpu_dm_crtc_atomic_get_property(struct drm_crtc *crtc,
 					    const struct drm_crtc_state *state,
 					    struct drm_property *property,
 					    uint64_t *val)